Radiation density gauge control of sludge transfer operations in sewage works



Aprll 14, 1964 c. o. BADGETT 3,128,786

RADIATION DENSITY GAUGE CONTROL OF` SLUDGE TRANSFER OPERATIONS IN SEWAGE WORKS Filed Nov. 27, 1959 4 Sheets-Sheet l CONTROLLER INVENTOR APllIl4, 1964 c. o. BADGETT 3,128,786

" RADIATION DENSITY GAUGE CONTROL OF SLUDGE.' v

TRANSFER OPERATIONS IN SEWAGE WORKS Filed Nov. 27, 1959 4 Sheets-Sheet 2 SERVO AMPLIFIER MEASBURING CALIBRATING CIRCUITS LIDS NT. so 42 L` PERCE leb/ INVENTOR C s C T IL IL T H N MOMC MAC O MRLA IMUT C wmm OMM O TM L U O U S A AC C A W April. 14, 1964 Filed NOV. 27, 1959 RADIAT TRANSFER OPERAT C. O. ION DENSITY 4 Sheets-Sheet 3 f l |45 94d i 94o I R l |520 l ly l WN 152e r INVENTOR April 14, 1964 c. o. BADGETT 3 l2 RADTATTON DENSITY GAUGE CONTROL OF SLUDGE 8786 TRANSFER OPERATIONS 1N sEwAGE wORKs Filed Nov. 27, 1959 4 Sheets-Shea?l 4 AIR wo. nbr. n

Denim n] PROPORTIONAL CONTROLLER 3M 3'8 I G )'-ICONTROLLERI 326 INTEGRATOR 302 H/BIO @L ZMM United States Patent Office 3,128,785 Patented Apr. 14, 1964 hio, assigner to Corporation, a corporation This invention relates to sewage treatment processes and the like, and more particularly it relates to a novel method and means for controlling sludge transfer operations so as to maintain optimum solids concentrations in various sections of the plant, thereby enabling the treatment processes to proceed at top efliciency.

v In a sewage treatment system, the principal processes are concerned with the separation of organic materials and inert solids from the aqueous carrier. Presedimentation, chemical precipitation, trickling filtration, activated sludge processes and the like are all accompanied by the settling out of solids to form sludge, which must be removed from its deposition place and either return to active circulation in the same process or removed to another process for disposal.

Asis Well known, it is a matter of great importance that sludge transfer operations should proceed when, and only when, the solids concentration of the sludge is within a certain optimum range. This becomes apparent, for example, in consideration of the characteristics of an anaerobic digester.

The anaerobic digester receives sludge from a settling tank, and, by a microbiological process, converts a major portion of the organic matter present to carbon dioxide and methane, leaving the solids content of the residue in a more purified condition. In order to achieve full utilization of digester capacity, theraw sludge should be introduced in the thickest possible condition. It has been found that it is possible to increase the concentration of this material into the range ofabout fifteen percent solids without any detrimental effect on digestion.

Assuming that at present the concentration of solids in the raw sludge introduced to digesters is about five percent, it is apparent that for every gallon of solid material processed, nineteen gallons of water `must be moved, heated if necessary to the proper digester operating temperature, and provided with digester storage room during the required detention period. Now if the concentration of solids can be increased to ten percent, only nine gallons of water must be so handled along with each gallon of solid waste. This means that a given amount o'f sewage can be processed in a plant having only half the original sludge pump and digester capacity, and even less than half the heat exchanger or boiler capacity. Since digesters and associated equipment on the average account for about one-quarter to one-third of the construction cost of a sewage treatment system, there is a very obvious economic advantage in properly controlled sludge transfer operations.

Y The addition of only uniformly thick sludge to a digester is advantageous inl other respects, as by minimizing the necessity for discharging excess quantities of supernatant liquor from the digester back into the sewage treatment process, thus avoiding the likelihood of malodorous conditions, or by reducing the necessary capacity of sludge concentration apparatus. Moreover, it results in an obvious saving of the space, equipment and operating expense associated with the eventual digested sludge dewatering process, since the required capacity thereof is dependent on the volume of liquid to be handled, regardless of the solids content.

Another example of the importance of solids-concentration-controlled sludge transfer is found in the activated sludge process, where activated sludge from the settling tank-must be recirculated back through the aeration tank in order to maintain the culture of aerobic microorganisms which effect the digestion of the organic materials. Here a proper balance between sludge return and sludge withdrawal must be maintained, so as to prevent depletion of the aerobia population on the one hand and to prevent excess sludge build-up and over-contamination of the efliuent on the other.

The above stated principles are well recognized, and their application has received the best efforts of sewage plant designers and operating personnel. However, due to the limitations of prior methods and means for determining and controlling the solids content `of `transferred sludge, the practical application of these principles has been beset by serious operating diliiculties which have rendered the attractive theoretical benefits largely unattainable in fact.

Prior to the present invention, sludge transfer operations have been regulated in a more or less unsatisfactory manner by the use of one or more of several techniques.

In one system, which is generally adapted only to small plants, the sludge is pumped into an open observation box before being transferred to the digester, so that the operator can estimate the solids content by stirring the sludge or merely watching the ow thereof. The results, however, are dependent on the subjective judgment of the operator, who is probably influenced more by the color of the sludge and variations in ambient light conditions than by the actual solids content. Although a fairly good measure of control can be achieved by this method with proper attention, it is very objectionable in that the sludge must be handled in the open, with the attendant odor problem and the necessity for continually washing down the observation box.

In an attempt to attain the benefits of direct observation of the sludge without handling it in the open, sight glasses have been installed in the sludge lines. However, being without the benefit of the feel of the stirrer or the opportunity to observe the behavior of the fluid under conditions of unconned ilow, the operator must be guided almost exclusively by color changes, and this is very unreliable since commonly the liquid sewage has a black and opaque appearance closely resembling that of the sludge. In many cases the behavior of the flow has been found to be a better indicator than the color, and this has led to the use of telescoping valves for observation of the flow.

It is the practice in some sewage plants to install small outlets in the sludge pipes, and to provide receptacles and catch basins thereunder, whereby a continual sampling of the sludge is made during the transfer thereof. Very accurate determinations of solids content can be made by evaporating the samples to dryness in the laboratory; however, the results are not usually available until twenty-four hours later, long after the transfer of the sludge has been completed and too late to utilize the results in controlling the current transfer operation. Hence for control purposes use is frequently made of the rapid centrifuge test, described by Symons and Torrey in Water Works and Sewage, March 1941, p. 106', which is probably satisfactory enough for rough plant work, but should be supplemented by actual solids determinations. While this method of controlling solids concentration is one of the best of the expedients previously employed, its accuracy and speed are quite limited, and it necessitates a continual clean-up operation.

Another expedient which has been employed to control sludge transfer is the use of programmed timers for alzarse actuating valves or turning pump motors on and olf at set intervals. This system suffers from the difficulty that the rate ofsludge deposition in settling tanks is not constant, and moreover the rate of ow through the pumps or gravity lines is variable vand dependent on the unknown consistency of the sludge.

In all the above described prior methods for controlling sludge transfer, for obvious reasons the operations are always conducted so as to allow a substantial margin of safety in contemplation of the ever-present possibility of the sludge becoming so dense and thick that a stoppage occurs in the sludge lines and/ or pumps. The natural consequence is that almost invariably too much liquid is transferred along with the sludge, with the unfavorable results which are apparent in View of the above discussion.

In accordance with the present invention, it has been found that sludge transfer operations can be effected in an accurate, reliable and automatic manner'under the control of a penetrative radiation device responsive to the density of the flowing sludge. A preferred form of the invention utilizes a source of penetrative radiation, such as gamma rays, located on one side of the sludge pipe, and a detector of said radiation, such as an ionization chamber, on the opposite side of the pipe. T he intensity of the rays penetrating the pipe and the sludge therein is variably attenuated in accordance with changes in the density of the sludge. The output of the radiation detector then provides an electrical signal which is processed by a suitable measuring instrument having an indicator or recorder to translate the signal into an appropriate reading in terms of density units or percent solids. The invention further provides a novel control system responsive to the indication of the radiation measuring instrument for automatically regulating the operation of the sludge transfer system.

It is the object of this invention to improve the eiciency and economy of sewage works and ,the like, by controlling the transfer of sludge in accordance with the solids content thereof as indicated by the density of the sludge.

It is also an object to provide a'method and means whereby a greater volume of sewage can be handled by a plant having agiven capacity of the sludge disposal apparatus.

' It is likewise an object to provide a method and means for controlling sludge transfer operations in a completely enclosed system, so as to minimize objectionable odors and the necessity for many'obnoxious clean-up operations.

It is another object to provide a method and means for continuously determining sludge solids content, in an automatic, reliable and instantaneous manner.V

It is still another object to eliminate the need for laborious and time-consuming laboratory analyses in the determination of sludge solids content.

It is yet another object to provide apparatus for automatically starting and'stopping the ow of sludge from a settling tank to a sludge processing apparatus so as to transfer only sludge having greater than a minimum solids content.

Itis a further object to provide means for automatically regulating the rate of ow of sludge in accordance with the solids content thereof.

It is a still further object to provide a method and means for maintaining theproper balance of sludge return and withdrawal in an activated sludge process, so as to maintain a peak population of aerobia without permitting the discharge of excessive solids in the eiiiuent.

It is an additional object to provide means for controlling the transfer of sludge having maximum solids content without undue risk of pump or sludge line stoppage.

Moreover, it is an object of this invention to provide apparatus in accordance with the above objects which is relatively inexpensive to build, easy to install with a minimum of modification to existing plant facilities, and which is reliable and economical in operation.

Other objects and advantages will become apparent in FIG. 3 is a detailed schematic of portions of FIG. l. showing of the basic form of one type of controller suitable for use in the system of FIG. l.

FIG. 4 is the electrical circuit diagram of a more elaborate controller, showing how the circuit of FIG. 3 may be modified to perform additional self-monitoring Y functions contributing to utility, efficiency and convenience of operation.

FIG. 5 is a schematic showing of an aerobic digestion process having an automatically controlled activated sludge balance system 1n accordance with the invention.

of the tank to form sludge 18a which is scraped into ay FIG. 6 illustrates one modification of the system of FlG. 5. Y

Referring to FIG. l, the numeral lil indicates a primary settling tank having an inlet pipe l2 for receiving raw sewage, for example, as it may be delivered from the screens and grit chambers (not shown). The sewage ows into a bafe chamber 14.-, thence over a Weir plate lo into the main settling tank, and out the opposite end (not shown). In the main settling tank, solid particles settle out of the quiescent liquid and drift to the bottom sludge hopper 20 by slow-moving wooden ights as at 22 carried on a conveyor chain 24.

In the sludge hopper 2t), the sludge 1% accumulates and becomes relatively packed at the bottom of the hopper, whereas at the top of the hopper the solids content is much decreased. Specifically, in general the solids content and density of the sludge continuously increases with the depth thereof in the hopper.

Whenever the sludge 16 has reached a certain solids content, it becomes necessary to transfer the same to sludge disposal means, here illustrated as an anaerobic digester 2,6. Commonly the transfer is effected by a sludge pump 28 having an inlet pipe arrangement 30 descending into the sludge hopper 20 and terminating the bottom thereof. The to the digester 26 by a is preferably driven by an with a suction ange 30a near pump outlet 28a is connected sludge pipe 32. The pump 28 electric motor 34.

In accordance with one preferred embodiment of this invention, a radiation density gauging head 36, to be described, is installed in the sludge transfer line and coupled by an electrical cable 38 to an electrical indicating device 4t?. Y The indicating device preferably comprises a continuously measuring recorder having an indicating pen and pointer arrangement 42 and an associated scale 44 which may be calibrated in terms of percent solids as shown. The recorder also preferably includes a setpoint indicating pointer 46, also cooperating with scale 44, and a set-point adjusting knob 8 mechanically coupled to the pointer 46 as indicated by the dotted line 50. The recorder 40 is connected via a multiconductor cable 52 to a controller 54, which regulates the application of electrical power to the pump motor 34, the power connections to the motor being indicated by line 56.

Referring now to FIG. 2, the radiation density gauging head 36 of FIG. 1, comprises a source 58 of penetrative radiation located on one side of a pipe section 60, and radiation detector 62 located on the oppositeV side thereof. A sealed capsule containing radioactive cesium-l37 has been used as the source 58 with very satisfactory results in practice; however, it will be appreciated that certain other types of penetrating ray sources such as X-ray, beta ray or neutron sources can be adapted to this purpose. Likewise an ionization chamber has been employed as the radiation detector 62, although a Geiger-Mueller tube, scintillation counter or other type of detector may be used. For radiation health safety, the source 5S should be weld shielded against the emanation of penetrative rays into the external environment, and to this end a block 64 of the rust-resisting iron alloy known as Meehanite has been used to surround the measuring area of the pipe section 60, with lead-filled caps 66 and 68 providing additional shielding along the axis of the cavities which contain the source and detector. It is seen that item 60 consists of an integral, unbroken section of pipe which completely isolates the radioactive source capsule from the flow of sludge through the gauge.

The radiation detector 62 is connected by conductors 70 to suitable measuring and Calibrating circuits 72, electrically associated with a rebalancing slidewire 74 which is driven by a servo motor 76 energized by a servo amplilier 78. The detailed structure and operation of the combination of items 72-78 is set forth in U.S. Patent No. 2,790,945, issued April 30, 1957, to H. R. Chope, and accordingly only brief mention of this apparatus need be made herein. In brief, however, the system functions as follows:

The sludge 18h within the pipe section 60 comprises water and a variable amount of solids. The latter, when in a dry state, are found to have a density of about 2.3-2.4 grams per cubic centimeter, compared to the essentially unit density of the water. Hence the mass of the sludge in the path of the penetrating rays between the source 58 and the detector 62 is proportional to solids content. Since the radiation travensing this path is Variably attenuated in dependence on the mass of the sludge, the signal output of the detector 62 Varies therewith. Calibrating circuits provided in box 72 are adapted to permit correlation of the variable signal with the actual solids content of the sludge 18!) as determined by laboratory analysis. When the instrument is suitably calibrated, the action of the servo motor 76 is to continuously drive the slidewire to a point of balance with the value of the detetor output signal, and, through mechanical connections indicated by the dotted lines 80 and 82, the recording indicator 42 is simultaneously driven to a point on scale 44 which indicates the instantaneous value of the solids content.

When the recorder 40 is adapted to one preferred form of the automatic controller 54, the servo motor 76 is arranged to drive a rotating cam 84 which is concentrically mounted with a cooperating switch plate 86. Secured to the switch plate is a suitable microswitch 88 having an arm 88a which bears against the periphery of the cam 84. The switch plate 86 carries a ring gear 86a which coacts with a spur gear 90. The spur gear is mechanically connected to the target pointer 46 and the set-point adjusting knob 48 so that when the knob 48 is manually adjusted to set the target pointer 46 in a desired position relative to scale 44, the switch plate 86 will be set in a corresponding angular position. Accordingly, as the recording indicator 42 moves relative to scale 44, when the percent solids indicator 42 is located to the right of target pointer 46, the contacts of microswitch 88 are open;

Vwhen the indicator 42 is located to the left of pointer 46, the contacts are closed. Via line 52a, the contacts of switch 88 are electrically connected into the circuits of the controller 54.

The controller may include la panel 54a for mounting a clock timer 92 and an interval timer 94 which cooperate with the microswitch 88 and other components of the controller to regulate the operation of the pump motor in a manner set forth in FIG. 3.

One form of the clock timer 92 comprises a twentyfour-hour clock dial 96 having an associated hour hand 9,8 carried on a shaft 100 driven by a synchronous motor 102 and gear box 104 arrangement through a drag clutch 106 which permits setting the `time of day by manual relocation of hour hand 9,8. To adapt the clock to perform a time switching function, it is provided with a selective commutator device including an integral pair of electrically conductive wipers 110 and 112 insulatingly secured to shaft and driven in fixed relation to hour hand 98. Moving wiper bears against a stationary metallic slip ring 114, while the other wiper 112 travels around the face of a stationary insulating ring 116 having a plurality of metallic contact buttons as at 118 molded therein. Each contact button is connected through an individual :switch as at 120 to a bus ring 122, the other switches not being shown in the drawing. The switches as at 120 may be of the simple pin jack variety and settable to open or closed position by means of small plunger knobs as at 120:1 located on the periphery of the clock dial 96 in line with the dial markers thereon. A circuit to be controlled is connected via a lead 124 to the bus ring 122, and via a lead 126 to the slip ring 114. Hence the external circuit, normally open, will be closed while the wiper 112 is passing over a contact button as at 11S when its associated plunger switch as at 120 has been set to closed position. For example, if switch plunger 120:1, at three oclock on the dial 96, is set to close its associated switch 120, an electrical contact between leads 124 and 126 will occur at three oclock am.

At 128 there is shown a D.C. power supply comprising a bridge rectifier energized from the conventional 110 v. A.C. power source 130 which also supplies power to lines 132 and 134 so as to drive the clock motor 102 and other circuits of the controller. The rectifier bridge 128 supplies D.C. lines 124e and 136.

In circuit with the D.C. power supply lines 124:1 and 136 and the clock timer 92 is a clock relay 138 with contacts 138:1 and 138]) which in turn are in circuit with a pulse relay 140 and a capacitor 142.

In circuit with the power supply lines 132 and 134 are a set of contacts 140:1 of the pulse relay 140, and a hold relay 144 having contacts 144:1. The circuit further includes the interval timer 94 of FIG. 2 which comprises a synchronous motor 94:1, an electric clutch 94h, and a trip mechanism 94e which controls switch contacts 94d and 94e. It is seen that contacts 94e are in parallel with the contacts 88b of the microswitch 88 (FIG. 2) which is associated with the recorder mechanism. Contacts 88b, 94e, 140:1 and 1.4451 are in circuit with a control relay 146.

The block diagram portion of FIG. 3 depicts the conv entional 'circuits of the pump motor 34 as modified for automatic control. These circuits may include a polyphase power source 148, motor relays and the regular manual controls 151. In accordance with this invention, an automatic-manual transfer switch 153 is preferably provided :so that the mode of motor operation can be transferred either to the manual control 151 or to the automatic control contacts 146e of the control relay 146.

The operation of the apparatus depicted in FIGS. l, 2 and 3 can now be described. Assuming that the sludge hopper 20 has been emptied of sludge, some time will elapse before the settlings from the tank 10 can build up a packed accumulation of dense sludge in the hopper. During this interval the sludge transfer system is at rest, awaiting the time when a sludgetransfer operation should be initiated. It is also assumed that this time is three oclock, and that the clock timer 92 has been set accordingly by means of switch knob 120a. At three oclock, then, the wiper 112 of the clock switch will make connection with the contact button 118.` A circuit will now be completed from the D.C. power supply 128 through line 124, bus ring 122, plunger switch 120, contact button 118, wipers 112 and"`110, slip ring 114, line 126 and clock relay 138 tothe opposite side 136 of the DC; power supply. Relay 138 will now be energized.

It is seen that capacitor 142 will have been connected through relay contacts 138:1 across DC. power supply lines 124:1 and 136 and charged thereby. When relay 138 4is energized, contacts 138:1 disconnect the charged capacitor from line 124e and contacts 138b will connect the capacitor across the coil 140 of the pulse relay. Thus the pulse relay will be momentarily energized, closing its contacts 14(ia. Shortly, capacitor 142 will discharge through relay coil 140 and contacts 144m will re-open.-

Hence it is seen that the sole function of the clocky timer 92 and its associated relays and D C. power supply is to cause the pulse relay contacts 141m to close momentarily whenever the hand 98 of the clock arrives at an hour which has been set by depressing the associated switch button on the clock dial.

The closure of the pulse relay contacts 141m energizes the hold relay 144, which locks in and holds through its own contacts 14411 and the interval timer contacts 94e. Therefore, even though the pulse relay contacts 14de re-open immediately, power will still be applied to line 145 and the hold relay 144 will remain energized. It is seen that the coil 146 of the control relay is connected in parallel with the coil 144 of the hold relay. Hence relay 146'wil1 operate its contacts 14641, which in turn cause the sludge pump motor 34 to operate by applying power thereto from the polyphase source 148 through the automatic-manual switch 153 andmotor relays 150.

Referring to FIG. 1, when the sludge pump 28 is iirst started, the suction pipe 3), the pump, and the density gauging head 36 will contain relatively thin and watery sludge, so that the recorder 41B will show a trace as at x indicating low solids content. Accordingly the microswitch 8S (FIG. 2) on the recorder will be open as indicated by the condition of contacts 83h in FIG. 3. It is the purpose of the interval timer 94 to provide the parallel connection through its contacts 94e so that power will remain on line 145 and thereby keep the control relayV 146 energized and the pump 34 running until thick sludge from the hopper is passing through the density gauging head 36 and producing a trace as at y on the recorder 40. This causes the microswitch $8 to close its contacts 88b as the measuring indicator the set point indicator 46.

The interval timer 94 (FIG. 2) is of a common type having a timing indicator 94f and a manually set interval selector 94g. When the sludge pump gizing line 145 (FIG. 3) power is applied to the synchronous motor 94a of the timer through the timer contacts 94d. Power is also applied directly to the electric clutch 9411 which connects the motor 94a to the trip mechanism 94C and also to the timing indicator 94f of FIG. 2 which slowly travels around the dial until it reaches the position of the interval selector indicator 94g. Thereupon the trip mechanism 94C is actuated, opening contacts 94d and 94e. The motor 94a stops, but the trip mechanism cannot reset while power remains applied to the clutch 94b from line 145.

As stated heretofore, 94 times out and opens its contacts 94e, thick sludge from the sludge hopper 20 will have traveled therefrom to the density gauging head 36 and increased the reading on the recorder so that microswitch contacts 88h will have closed before the timer contacts 94e open. Hence the holding circuit for the hold relay 144, the timer clutch 94h and the control relay 146 is completed solely through the density recorder microswitch. As the sludge pump 28 continues to operate, withdrawing the sludge 1S out of the hopper 20, the solids content of the sludge will gradually decrease as shown by the recorder trace of FIG. l. When the indicated solids content has dropped below the set point indicated by the position of pointer 46, the microswitch contacts Sb open, removing power from line 145. Hold relay 144, timer clutch 94b and control relay 146 will be de-energized, the timer 94 will p be reset, the pump motor 34 will stop, and all the original circuit conditions will again prevail.

42 passes to the right ofv is started by ener-y by the time the interval timer` g, v The apparatus described in connectio 3 is a fully operative embodiment of the invention in one form thereof. FIG. 4

and useful additions to the circuit of FIG. 3. These devices permit the sludge transfer system to be placed in fully automatic operation in a shorter period of time, and provide automatic trouble-shooting features.

The rst device is represented by the additional timer indicated by the dotted lines in FIG. 2. This timer is designed to shut olf the sludge pump in the event that the density meter system has permitted the pump to run longer than a predetermined maximum period of time. If this occurs, it

to this end a warning light indicated by the dotted lines 154 may be placed on rence requiring attention is in the case Where the pump shuts off immediately after the minimum pumping interval timer 94 has timed out. is signaled by a further warning light indicated by the dotted lines 156. A pushbutton indicated by the dotted lines 158 is provided to reset the alarm signal light 154 and/or 156 before or after the difculty has been investigated.

Referring to FIG. 4, in light lines, whereas the modications are shown by the heavy black lines. The-maximum on time timer 152 is of the same type as the minimum on time timer 94, and comprises a synchronous motor 152a, a clutch 152b,

atrip mechanism 152C and three sets of contacts 152d,`

152e and 1521 The timer motor 15211 is connected to line 145 through the normally closed timer contacts 152d; the clutch 1521) is connected directly to line 145, and the normally closed timer contacts series with the contacts 8817 of density gauge recorder.

The alarm signal lights 154 and 156 are connected in circuit with a normally closed pushbutton switch 158 across lines 132 and 134. A hold relay 160 is connected in parallel with the light 154, and a normally open set of contacts 16% of the hold relayV 160 is`connected in parallel with Vthe contacts 152]c of the timer 152. Another hold relay 162 is connected in parallel with the light 156 and a set of normally open contacts 162a of hold relay 162 Vconnect thercombin'ation to line 32 through the pushbutton switch 153. A parallel circuit across contacts 162a includes an extra normally closed set of contacts 144b on hold relay 144, whose operation has been described, and a normally open set of contacts 164g of a time delay relay 164. The operating circuit of the time delay relay is connected in parallel with the synchronous motor 94a of the minimum on time timer 94.

The operation of the circuit as the microswitch on the described in connection pump motor is shut off immediately when the timer 94 times out, or (2) in the case where the pump motor is still in operation at the end of a predetermined maximum p interval which is set on timer 152.

Assuming that the sludge pump motor has been started as hereinabove described in connection with FIG. 3, it is seen that when power is applied to line the normally closed contacts 14411 of the hold relay 144 will open. The timer clutch 152b as well as the timer clutch 94b will be energized, and the timer motor 152a as well as timer motor 94a will start running. The time delay relay 164 will be energized in parallel with the timer motor 94a, and after a short interval the time delay relay contacts 164a will close. However, the light 156 and relay 162 will not be energized becausercontacts 14411 are open.

In a normal situation, when the timer 94 times out and opens its contacts 94d and 94e, the density gauge microswitch Vcontacts 88h Will have closed to maintain the'holding circuit for relay 144. In this case, power remains on line 145 and the sludge pumpcontinues to run. However, the opening of timer contacts 94d removes power with FIGS. 1 to illustrates a'circuit including novel v 152 on the control panel 54a` is desirable thatV A the plant operating personnel be apprised of the fact, and

the control panel. Another occurp Accordingly this occurrence p the circuits of FIG. 3 are redrawnA 152e are connected inv from the time delay relay 164, and after a-short interval of delay the contacts 16411 thereof will re-open. Accordiugly when the microswitch contacts 88h remove power from line 145 and de-energize relay 144, the re-closing of its contacts 144b will not be able to energize the signal light 156.

On the other hand, consider an abnormal situation, for example, where the clock timer 92 has been improperly set so as to turn on the sludge pump at too-frequent intervals and the density of the sludge is initially less than the cut-oit value. When the interval timer 94 times out and opens its contacts 94d and 94e, the density gauge microswitch contacts 88b will be open, power will be removed from line 145 immediately, and relay 144 will be deenergized and re-close its contacts 144b. In this case, due to the time delay action of relay 164 its contacts 164a will still be closed, and power will be applied to the signal light 156 and hold relay 162. Relay 162 will thereupon establish a holding circuit through its own contacts 162a and remain energized together with the light 156 until the holding circuit is broken by manual operation of pushbutton switch 158.

It is apparent that the alarm system utilizing signal lamp 156 is also eifective to indicate the presence of any malfunction in the density gauge and recorder system which would prevent the instrument from properly closing the microswitch contacts 88k.

Referring now to the operation of the maximum on time timer 152, consider a normal situation wherein the sludge pump has been started and after its set interval the timer 94 has timed out, opening its contacts 94e, The holding circuit for relay 144 is completed through its contacts 144er, contacts` 88b of the density recorder microswitch and contacts 152e of timer 152. Ordinarily, the sludge transfer operation will be completed when this holding circuit is opened by contacts 88h. As a result of the removal of power from line 145 and the clutch 152]), the timer 152 simply resets itself without any eiect on the controller operation.

However, in an abnormal situation opposite to that just described, where for some reason the density gauge microswitch contacts SSb do not open and thereby shut oif the sludge pump within a maximum allowable pumping time as set on timer 152, this timer will time out, opening its contacts 152e and thereby shutting oif the sludge pump. This occurrence is signaled by alarm light 154, since the closure of timer contacts 152)c will energize the same together with hold relay 160 which will establish a holding circuit for its coil 16) and the lamp 154 through contacts 169.1. This holding circuit will be maintained until reset pushbutton 158 is manually depressed.

It may be desired in the last-mentioned situation to allow the pumping to continue after the alarm has been triggered, and if so, the function of contacts 152e can be dispensed with. Thus contacts 152d are employed to stop the timer motor 15261 when the timer times out. `These contacts also serve this purpose when contacts 152e are in the circuit, in case for some reason timer 94 fails to open its contacts 94e. Such a failure will of course be indicated by operation of the alarm signal lamp 154.

The proper setting of timer 152 must generally be determined by keeping records of the length of the pumping intervals over a period of time, but when the setting is found the alarm function is useful as an indicator of the possible need for a resetting of the clock timer 92 so as to initiate the pump cycle more frequently, for example, during a rainy season when increased sewage ilow and higher flow velocities in tributary systems bring about a more than proportional increase in daily sludge deposition.

In FIG. there is shown an aeration tank 200 receiving settled sewage through a conduit 202 which may be connected to the eflluent end ofthe settling tank of FIGQL The culture of aerobic bacteria which is maintained in tank 209 is supplied with air through a conduit 204 extending to the bottom of the tank and connected to a distribution system 2,06 having a network of metering orices for generating suitably sized air bubbles which rise through the sewage and aerate the same. From the aeration tank the sewage ilows into a final settling tank 208 and 4afterwards the final eiuent thereof is discharged at the outlet 210 lof the sewage works.

The activated sludge 212 which settles out of tank 2&8` is withdrawn periodically or continuously through a sludge pipe 214, either by gravity ilow or through the agency of a pump 215. The sludge pipe 214 has two branches, one of which, shown at 216, leads to sludge disposal means such as an anaerobic digester, whereas the other branch 218 returns the activated sludge to the aeration tank 200 to maintain the population of the aerobic bacteria therein which digest the organic matter in the sewage. Each of the branch lines 216 and 218 of sludge piping is headed by a Valve as indicated respectively at 220 and 222 whereby the balance of sludge withdrawal and sludge return can be adjusted.

In accordance with this invention, said balance is maintained by automatically controlling the positioning of valves 229 and 222 in accordance with a density indication provided as hereinabove described. To this end, the radiation source 58 and detector 62 are placed adjacent a section of the common sludge pipe 214. The indicating pen and pointer 42 of the recorder 4i) are mechanically connected to the variable tap 224m of a repeat slidewire 224. The set-point indicator 46 is mechanically connected to the variable tap 226e of a potentiometer 226. Potentiometers 224 and 226` are connected into a bridge circuit energized by a voltage source 23S through a Voltage dropping rheostat 230. The taps 224e and 226e are connected to the input or a conventional proportional controller 230 which controls the application of power to an electric motor 232. The motor 252 drives the valves 22@ and 222 through a gear box 234. The gear box is arranged so that when motor 232 runs in one direction, valve 2241 is driven toward its closed position and simultaneously valve 222 is driven toward its open position. When the motor 232 is reversed, the opposite effect obtains. The gear box output also drives a suitable potentiometer 236 in the circuit of the controller 230 whereby an electrical voltage` representing the position of the valves is fed back to the error sensing element (not shown) of the controller, Since the control system 224-236 is of rather conventional design, a more detailed description thereof is deemed unnecessary herein.

One way in which Vthe apparatus of FIG. 5 may be adjusted is such that when the indicators 42 and 46 are in vertical alignment there will be no potential difierence between `the taps 224e and 226:1, that is, the input signal to the controller will be zero. At the same time, when the valves 220 and 222 have a particular setting, say, each valve half open, the potentiometer 236 will be set to deliver zero feedback voltage to .eifect balance in the controller and cause the motor 232 to remain at rest.

Now if the position oi recorder indicator 42 deviates from the position of the set-point indicator 46, a deviation voltage will be developed between taps 224a and 226a of the bridge potentiometers. The polarity of this deviation voltage will depend on the direction of the indicator deviation, and the magnitude will be proportional to the amount of the indicator deviation and the setting of rheostat 230. In accordance with the polarity of the deviation voltage, the motor 232 will operate in the proper direction to drive the valves to a new position, wherein the feedback voltage from potentiometer 236 is equal and opposite to the deviation voltage.

It is seen that by the control system of FIG. 5 the flow of activated sludge returned through pipe 218 is maintained inversely proportional to the density of the sludge.

aiasfisa Referring Vnow to FIG. 6, there is shown a further embodiment of the invention wherein a settling tank 300 is provided with a sludge well 392 and a conventional telescoping valve 304. Valve 3M includes an outer pipe 304er extending downwardly into the sludge hopper 306 and sealingly secured into the iioor of the sludge well 302. An inner pipe 304]? is slidingly mounted inside the outer pipe 30K-la and attached at its upper end to a spider 3% and lifting rod 310. The sludge flows upwardly through the telescoping valve and spills over the top of the inner pipe 3tl4b at a rate dependent on the difference in height between the top of the movable pipe and the surface of the liquid in the settling tank Silit. Accordingly the flow rate can be controlled by moving the rod 31u up or down. In this system, the withdrawal of the sludge takes place continuously, and the density of the transferred sludge is dependent on the rate of withdrawal thereof iu relation tothe deposition rate. The radiation source 58 and detector 62 are located adjacent the pipe 312 which drains the sludge well 392. The controller 314 employed in this instance is of a reset type such as disclosed in FIG. 2 of U.S. Patent No. 2,895,888 issued July 21, 1959to Donald E. Varner. The actuator motor 316 is employed to reposition the telescoping Valve lifter rod 316 through a gear arrangement 318.

, The invention further contemplates the combination of the radiation measuring system with a suitable owmeter to indicate or control the mass flow rate of sludge solids transferred. To this end a suitable flowmeter 320, for

example, a magnetic flowmeter such as is manufactured and marketed by the Foxboro Company, is installed in the sludge line 312 and connected to a recorder 322. The recorders 40 and 322 ,are associated with a suitable computer 324 for multiplying the percent solids reading of recorder 4i) by the ow rate reading of recorder 322 to derive an indication of the weight of dry solids transferred per unit time. For example, the two recorders maybe equipped with repeat slidewires connected in the,

circuits of a conventional servo potentiometer multiplier which continuously records the dry solids transferA Likewise the multipler 324 is adapted to provideV rate. an input signal to a suitable integrator 326, which may be of the type described in US. Patent No. 2,513,537 issued Iuly 4, 1950, to F. C. Williams. The integrator is calibrated to continuously indicate the total weight of dry solids transferred.

It can be seen that the systemV of FIG. 6 is also useful for purposes such as the accurate loading of a digester with a specied number of pounds of dry solids, dilutedy with a minimum amount of water.

. While the invention has been described and illustrated as being embodied in certain specific procedures and apparatus whereby the objects of the invention are fully accomplished, it is apparent that many modifications can be made to the disclosed procedures and apparatus, and many outwardly similar or quite different embodiments can be made, without departing from the spirit and ,A of said ow to provide i from a settling tank for directing a beam of 12 r scope of the invention as is set forth in claims.

What is claimed is:

1. Apparatus for controlling the removal of sludge so as to Withdraw therefrom only sludge having va desired solids content, which comprises sludge withdrawal means in connection withsaid settling tank for providing a floW of sludge from said tank, means penetrative radiation selected from the group consisting of gamma rays, X rays, beta rays and neutrons across the path of said flow, means for quantitatively detecting a portion of said radiation which has been modiiied by interaction with the mass an indication of the density of said flow, means for initiating operation of said sludge withdrawal means to start said ilow, means for maintaining said'operation for an interval not less than the time required for said flow of sludge to travel the distance from said settling tank to said detecting means regardless of said density indication, means responsive to said density indication for terminating said operation after the lapseof said interval when said density indication has been reduced to a selected value, and alarm signal means activated by failure of said density indication responsive means to terminate said operation within a pre-determined period of time after said lapse of said interval.

2. Apparatus for controlling the removal of sludge from a settling tank so as to withdraw therefrom only sludge having a desired solids content, which comprises sludge withdrawal means in connection with said settling tank for providing a 110W of sludge from said tank, means for directing a beam of penetrative radiation selected from the group consisting ot' gamma rays, X rays, beta rays and neutrons across the path of said flow, means for quantitatively detecting a portion of said radiation which has been modified by interaction with the mass of said flow to provide an indication of the density of said ow, means for initiating operation of said Vsludge withdrawal means to start ,said iiow, means for mainsaid density indication for further maintaining said optaining said operation for an interval not less than the time requiredfor said W of sludge to travel the distance from said settling tank to said detecting means regardless of said density indication, means responsive to eration after the lapse of said interval Whenever said density indication exceeds a selected value, and alarm signal means activated by failure of said density indication responsive Vmeans to maintain said operation after said lapse of said interval.

References Cited in the tile of this patent UNITED STATES PATENTS 2,454,653 Kamp Nov. 23, 1948 2,586,447 Way Feb.V 19, 1952 2,661,550 Graham Dec. 8, 1953 2,812,773 McGee Nov. 12, 1957 2,826,913 Rosenberger Mar. 18, 1958 2,911,826 KIitz Nov. l0, 1959 the appended 

1. APPARATUS FOR CONTROLLING THE REMOVAL OF SLUDGE FROM A SETTLING TANK SO AS TO WITHDRAW THEREFROM ONLY SLUDGE HAVING A DESIRED SOLIDS CONTENT, WHICH COMPRISES SLUDGE WITHDRAWAL MEANS IN CONNECTION WITH SAID SETTLING TANK FOR PROVIDING A FLOW OF SLUDGE FROM SAID TANK, MEANS FOR DIRECTING A BEAM OF PENETRATIVE RADIATION SELECTED FROM THE GROUP CONSISTING OF GAMMA RAYS, X RAYS, BETA RAYS AND NEUTRONS ACROSS THE PATH OF SAID FLOW, MEANS FOR QUANTITATIVELY DETECTING A PORTION OF SAID RADIATION WHICH HAS BEEN MODIFIED BY INTERACTION WITH THE MASS OF SAID FLOW TO PROVIDE AN INDICATION OF THE DENSITY OF SAID FLOW, MEANS FOR INITIATING OPERATION OF SAID SLUDGE WITHDRAWAL MEANS TO START SAID FLOW, MEANS FOR MAINTAINING SAID OPERATION FOR AN INTERVAL NOT LESS THAN THE TIME REQUIRED FOR SAID FLOW OF SLUDGE TO TRAVEL THE DISTANCE FROM SAID SETTLING TANK TO SAID DETECTING MEANS REGARDLESS OF SAID DENSITY INDICATION, MEANS RESPONSIVE TO SAID DENSITY INDICATION FOR TERMINATING SAID OPERATION AFTER THE LAPSE OF SAID INTERVAL WHEN SAID DENSITY INDICATION HAS BEEN REDUCED TO A SELECTED VALUE, AND ALARM SIGNAL MEANS ACTIVATED BY FAILURE OF SAID DENSITY INDICATION RESPONSIVE MEANS TO TERMINATE SAID OPERATION WITHIN A PRE-DETERMINED PERIOD OF TIME AFTER SAID LAPSE OF SAID INTERVAL. 